Different insurance companies have different underwriting rules. It may be that your company has a rule against having livestock and they may consider poultry livestock. It most likely has to do with liability concerns of someone getting sick via salmonella or the possibility of more than the average number of persons being on your property to purchase eggs, chicks, etc. It may be that they consider having chickens a business exposure as you have the possibility of selling eggs [even if you don't]. Yet another insurance company may not have any problem whatsoever with it.
I'd suggest you look for a carrier that writes not only homeowners policies but also farm owners policies. I doubt that you'd be in the "farm owner" category but companies that write farm owners are usually more pet and livestock "friendly" as they've underwritten the exposure before and are familiar with the risks - and non-risks - associated with having chickens. Your local Farm Bureau would be a good spot to start, if they don't have an insurance exchange they can probably recommend someone to you.
One of your chickens may attack someone causing injury--never can be too careful. <Done tongue in cheek> Insurance companies like to change the rules so they can cut the cost of claims and/or raise the premiums.
Call Nationwide. Don't mention the chickens unless they ask. They will cover the coop as long as you're not using it for a business..ie: selling chickens.